Means for suppression of noise in communication systems



April 25, 1950 s. T. FISHER 2,504,971

- MEANS FOR SUPPRESSION OF NOISE IN COMMUNICATION SYSTEMS Filed Nov. 8,1946 3 sheets sheet 1 m Z I 6 April 25, 1950 s. T. FISHER MEANS FORSUPPRESSION 0F NQISE m COMMUNICATION SYSTEMS 3 Sheets-Sheet 2 Filed Nov.8, 1946 J72 d6)? Z37 15 5 Ka /72g April 25, 1950 s. T. FISHER 2,504,971MEANS FOR SUPPRESSION OF NOISE IN COMMUNICATION SYSTEMS Filed Nov. 8,1946 3 Sheets-Sheet 3 26/4136 gain 7 l2 t 7a UNITED STAT MEANS FORSUPPRESSION OF'NOISE IN COMMUNICATION SYSTEMS Sidney 'i. Fisher,Montreal, Quebec, Canada, assignor to Federal Telephone and RadioCorporation, New York, N. YL, a corporation of DelawareApplicationNovember 8, 1946, Serial No; 708,569 In Canada March 16;194%- I Claim; (Cl. 179 1) TENT OFFICE 1 2 This invention relates to amethod of control Fig. 3 shows the gain-input level curve of the lingand diminishing undesired extraneous noises expansion amplifier. andsounds that may be impressed. on the micro- Fig. 4 shows thecharacteristic envelope curves phone of a communication system duringsuch for the noise and signal sounds as received by time as thetransmission of spe is g made the microphone and as received, by thereceiver.

over and by means of uc transmission Sy t Fig. 2 is a simplepresentation of the main fea- Volume expansion, I have discovered, is antures of the invention and it would perhaps coneifective and novelmethod whereby most undetribute to a better understanding of theinvensired sounds and noises can be reduced and. subtion to considerthis figure and Fig. 3 before exstantially eliminated while the spokenword is plaining in detail the circuit diagram of Fig. l. transmitted ina clear manner. In this drawing as represents voice sound waves imp v min the ad ption of voi such impression of 43 on so a different set ofsounds which emanate from sources or points sound waves 55 are alsobeing impressed on the where other sounds and noises are much in evi- I5microphone 44. It is to be understood, for pur dence. Such sourcesinclude the interior of poses of this description, that the sound wavesm ary tanks in operation, ocomotive cabs and it are of an extraneousnature and are not dethe interior of airplanes during flight. This novelsired i t transmission of t speech sound system of volume expansion willalso improve wav 43,

radio recept on w the transmitted D The electrical. impulses, set up bythis dual set grammes origina e in the vicinity of perating of soundwaves, pas from the microphone 4G to machin r or a ny place or l ationwher an amplifier 45. The out ut of the amplifier work is in prog e s, Sc Work ting no es enters an expansion amplifier 48 by two channels.

and sounds which may be consi ered 0 b d fi n A portion of the amplifieroutput from 45 goes, mental to the normal transmission of voice or bymeans of the circuit shown, to a rectifier 41 music over radiotransmitting systems. and thence to the expansion amplifier 48. The

' The p ati asi f th n v l app at on f balance of the output from 45goes directly to Volume expansion depends upon the established theexpansion amplifier 48.

fact that sound waves arising from the operation From the expansionamplifier 48 the impulses of machinery and mechanical devices, t ru 30are fed to an antenna and the ground and by of wind and the escape ofsteam are characterized m a of th usual transmission medium reach yhaving a relatively constant value and by a receiver 49 and loadcircuits as shown at 50.

possessing a peak value somewhat below the peak By use of this circuitit is found that the revalue of normal speech level. sultant voltagecoming from the rectifier 4! is Consideration and control of these twosound free from speech components and varies only at levels form thebasis of the invention. This conthe syllabic frequency of the speech.This volttrol is brought about by means of a novel transage output isused to control the gain of the mission circuit. expansion amplifier 48.

All of the above noted desirable features and Fig. 3 shows thegain-input level curve of a results and others which will hereinafter beap- 4.0 typical expansion amplifier. From this curve it parent areaccomplished by means of the novel will be seen that the gain increaseswith the arrangement of circuits and apparatus diagram- 111191.11; l-

matically illustrated in the accompanying draw- 8 1 shows e embodiment fthe invention ings forming a part of this p ifi ti and in a typicalradio telephone transmitter. A

microphone I is understood to be located at a w w m yp r1 .m nt sho mgby W of pleat lcal e bodl e point where undesired noises are of a volumeu 1 g ggl g g drawings, suflicient to interfere with the normaltransmission of speech sounds. As shown, the micro- Fig. 1 illustratesdiagrammatically the circults phone I is coupled to an input transformer2,

and apparatus of a transmitter equipped to carry 50 the secondary ofwhich leads to an expansion out the inventionamplifier stage 5 through acapacitor 3 across Fig. 2 illustrates 1n block diagram the arrangea gridresistor 4 merit, relation and connections of the various Amplifierstage 5 is capacity-resistance coupled units of the invention at atransmitting and reto an amplifier stage It by means of a capacitorceiving station. 9 and a grid resistor IS. The amplifier tube l0 Oneobject of the invention is t0 pe m all being impressed ona microphone44.. During.

has a cathode resistor H and a cathode by-pass capacitor I2, by means ofwhich the cathode of tube 5 is maintained at a substantially constantpositive potential with respect to the ground.

An output transformer for the second amplifler tube It] is indicated atl3. This transformer has an extra secondary winding [4, the voltage fromwhich winding is delivered to a load resistor l5 and an audio-frequencyby-pass capacitor H. The positive bias generated across resistor I6 isused to supply grid bias to the expansion amplifier 5.

The main portion of the power output from tube 19 is used to platemodulate a class C, radiofrequency amplifier tube 30 which is coupled byreactances 34, 35, 38 and 31 to an antenna 40. An adjustable reactance39 is connected through a radio-frequency ammeter designated by 38. Tube39 is supplied carrier energy from a master oscillator tube 23, with anoscillating circuit made up from a variable reactance l9, and a splitinductor 20. There are also two screen by-pass capacitors indicated by2| and 24.

The C0up1il1g circuit to the tube as is formed by a radio-frequencychoke 26, a variable condenser 21, two by-pass capacitors 28 and ti, anda grid return resistor 29.

The master oscillator tube filament 23 is connected to the groundthrough a radio-frequency choke 42.

In Fig. 4 are shown characteristic envelopes for sounds as received bythe microphone l; and as received by the receiver circuit 8 in Fig. 1.

A in Fig. 4 represents the envelope of the speech wave and B representsthe envelope of the noise wave. The ratio of these two waves is thesignal to noise ratio. It is seen to be poor during periods of lowspeech level.

Curve C shows the variation in amplifier gain brought about by thespeech wave. D represents the speech wave at the output of theamplifier. E represents the noise wave at the output of the amplifier.

The ratio of A to E is the signal to noise ratio at the output of theamplifier. A comparison of these sound envelope curves will indicate themanner in which the undesired noises and sounds are modified by theinvention.

From the foregoing it will be apparent that the invention will improvethe transmission and reception of voice signals in radio transmission,under all conditions wherein the voice signal originates from sourcessubject to undesired noises and sounds and that the novel method of soimproving such transmission of voice signals is brought about by passingthe combined signal and noise waves through a section of the circuit ofwhich the transmission efficiency increases with the signal and noisecombined level.

While the invention has been described in terms of a preferableembodiment thereof, it is not to be limited in terms of the embodimentdisclosed, but only in terms of the claim here appended.

A noise suppression system comprising a pick up influenced by bothsignal and noise energy, a volume expansion amplifier, a thermonic amplifier, an amplifier biased to cut-off, said amplifiers being connectedin series in the order named and to said pick-up, and means including arectifier connected between the output of said thermionic amplifier anda grid of said volume expansion amplifier for increasing the gainthereof with increased energy, and means whereby said amplifier isbiased to cut-off passing energy only when there is signal energysuperimposed on the noise energy.

SIDNEY T. FISHER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

